We study early events of lymphoid development, in particular gene rearrangement of the T cell receptor genes. Crosslinking of the Fc receptor on thymic precursors has been previosly found to block gene rearrangement and subsequently T cell development. We have now cloned thymic precursor cells and shown that the progeny of a single thymic precursor can be diverted into either the T cell lineage or natural killer cell lineage. We have previously identified a nuclear protein that binds flanking sequences on all rearrranging genes. We have purified the protein and are attempting to derive amino acid sequences from it. We have recently cloned a novel putative helicase that is specifically expressed in early thymic precursors. We are testing with the use of mutant expression vectors a role in gene rearrangement and T cell commitment. We have isolated genomic clones in order to study genomic structure and the promoter region of the lymphoid specific helicase. We are currently developing a targeted disruption of the helicase gene in mice in order to study a role for the helicase in lymphoid development. We are also studying the molecular mechanism of double-stranded DNA repair. The Ku-dimer is essential for gene rearrangement as well as DNA repair after radiation damage. We have tested the Ku86 and Ku70 peptides for protein-protein interaction in the yeast-two-hybrid system and are currently generating mutants. We have screened a Hela cell library for novel proteins interacting with the Ku86/Ku70 dimer. We are currently testing a number of isolated clones for specific interaction with the Ku-dimer to determine the role in the cellular response to radiation damage and double-stranded DNA break repair.